It is required for a synchronous motor particularly used in a compressor, an electrical vehicle, a hybrid vehicle, a fuel-cell-powered vehicle, and such to generate high torque with a low ripple, due to a demand for reduction in size, vibration, and noise, and improvement in output and efficiency.
In a surface magnet synchronous motor in which permanent magnets are distributed on the surface of an iron core, torque generated by the permanent magnets (i.e. magnet torque) is maximized when a phase difference between a field generated by the permanent magnets and an armature current is 90 degrees. In other words, the magnet torque is maximized if a current supplied to a stator coil is maximized when a point between magnet poles of the rotor and a stator tooth wound with the stator coil are aligned. The torque decreases as the phase difference between the field generated by the permanent magnets and the armature current is deviated from 90 degrees.
In an interior permanent magnet synchronous motor in which permanent magnets are embedded in an iron core, the following two types of torque are generated. One is magnet torque generated by the permanent magnets. The other is reluctance torque resulting from saliency due to a difference in magnetic resistance that is determined by positions of a rotor and a stator. The reluctance torque is maximized when a phase difference between the field generated by the permanent magnets and an armature current is approximately 45 degrees. Accordingly, the interior permanent magnet synchronous motor yields torque which is the sum of the magnet and the reluctance torque. The total torque is maximized when the phase difference between the field and the armature current is approximately 0 to 45 degrees.
Generally speaking, torque generated in a synchronous motor has ripple components generated under influence of a harmonic wave component of a field generated by a permanent magnet and of a harmonic wave component of an armature current. In an attempt to reduce the torque components, a technique has been conceived for adjusting the positions of stator coils supplied with currents of the same phase such that the intervals (angles) between the stator coils are mechanically offset relative to intervals (angles) between magnet poles of a rotor. This makes torque ripples generated in the respective stator coils out of phase from each other. As a result, the torque ripples are cancelled out by each other, whereby vibration and noise is reduced (Patent Literatures 1 and 2).
Patent Literature 1 discloses a synchronous motor in which a stator coil is wound around one stator tooth (i.e. concentrated winding type). In the synchronous motor, the number of magnet poles of the rotor is set to be 10, and the number of stator teeth is set to be 12. As for the stator teeth, two stator teeth group sets each composed of U+ phase, U− phase, V+ phase, V− phase, W+ phase, and W− phase are arranged in the stated order. In this case, stator coils supplied with a current of the same phase (e.g. U+ phase and U− phase) are arranged with an offset of π/6 electrical radians. This means that torque ripples generated in the respective stator coils are out of phase from each other by π/6 radians. Consequently, the torque ripples are reduced.
Patent Literature 2 discloses a technique for reducing cogging torque, which is a torque ripple generated in a state where a current is not supplied. According to the technique, cogging torque is reduced by setting a relation between the number of slots (i.e. teeth) around which stator coils are disposed and the number of magnet poles of a rotor to be 18 to 20, compared with a conventional synchronous motor with 12 slots to 8 magnet poles or with 9 slots to 8 magnet poles.